A typical radio transmitter of a mobile communication system comprises baseband signal processing stages, an RF (radio frequency) up-converter, several amplification stages operating at RF frequencies and a following RF power amplifier stage (which may comprise one or more power amplifiers).
Such radio transmitters for mobile communications systems must be capable of setting the RF output power across a wide dynamic range. For example, a 3GPP (3rd Generation Partnership Project) WCDMA (wideband code division multiple access) transmitter must provide a power control range of >70 dB, typically from around +23 dBm down to around −50 dBm.
An envelope tracking radio transmitter operates by carefully controlling the supply voltage to the power amplifier stage, such that the supply voltage tracks the instantaneous RF envelope of the signal to be amplified with high accuracy. The instantaneous signal envelope is used to derive the power amplifier supply voltage. The mapping between signal envelope and supply voltage is designed to achieve certain performance objectives. This mapping is implemented using a non-linear transfer function, often termed a ‘shaping table’
To date envelope tracking power supply systems for power amplifiers have focused on applications operating with a low power control range.
It is an aim of the invention to provide an improved envelope tracking power supply which provides for efficient operation with a high power control range.